Architect Steven Harris and interior designer Lucien Rees Roberts are a study in contrasts. Harris is quick to jump in and finish his companion's sentences, while Rees Roberts will wait and elegantly elaborate after the other man is done talking. The Florida-born architect's sensibility is lively and curious; the British designer is soft-spoken and reserved. But when it came time for the life partners to build a weekend retreat for themselves, the admired New York City–based tastemakers—each has an eponymous firm, and Harris is also a professor at Yale—were pretty much in sync. As Rees Roberts explains, "We wanted the country house to be casual and unself-conscious." Harris adds, "It's important that what we design reflect how somebody truly lives. So this is not a prototype for what we'd do for a client."

In the search for land upstate, Rees Roberts studied the topography shown on U.S. Geological Survey maps for an area with open fields. He found 50 available acres at the crown of a hill between the Catskills and the Berkshires and cut out a flat place for a building site. As a birthday present, Harris gave Rees Roberts a day with a rented bulldozer and Margie Ruddick, a New York landscape designer and frequent collaborator, to regrade the land and carve an unpaved drive in lazy curves that would allow the house they planned to "disappear and reappear," Rees Roberts says. "There's an intimacy in how the contours move back and forth from mountain views to open fields that reminds me of English landscape paintings."

Architecturally, Harris says he was drawn to the 1920s modernist concept of small spaces crafted as precisely as possible for maximum living. The first structure the pair built—a white-stained, shingled box completed in 1992, measuring 40' long by 14' wide by 20' high, and one room deep—stands east-west in what the architect calls "a Jeffersonian orientation in a lyrical landscape." A second building the same size was completed four years ago. Connected by an underground passage, it stands perpendicular to its twin and is oriented north-south, so the structures form an L. Exemplifying the grandeur possible in the phrase Less is more, the couple's residence seems impossibly spacious, with eight rooms and four baths on multiple levels. "The buildings are almost scaleless," Harris observes. "The windows are intentionally too big or too little, either picture frames or whole walls of glass."

Balancing his partner's academic viewpoint are Rees Roberts's stylish but practical interiors ("my idea of comfort is a good lamp to read by," he notes). On the vaulted ground floor of the east-west building is a large pine-walled room with a living area at one end; there, a graceful early-19th-century daybed and mustard-color linen sofa contrast with a fireplace made of unfinished concrete blocks. Tucked beneath the rafters is a little window positioned to capture the first light of sunrise. At the other end of the space, separated by stairs leading to a guest suite below, is a studio for Rees Roberts, a third-generation painter. "It's an incredibly quiet place to work," the designer says. "You only hear geese during the day and coyotes at night."

The dining room is located in the north-south building, opposite the living room, on the other side of a terrace planted with thyme. Fourteen-foot glass pocket doors and sliding screens transform the room into a breezeway, Harris says, that recalls the Southern dogtrot houses photographed by Walker Evans in the 1941 book Let Us Now Praise Famous Men. When the doors are opened wide, herb-scented breezes tinkle the dangling glass crystals of the '60s Venini chandelier.

Adjacent to the dining room is a gnarled apple tree Harris found in a nursery near the racetrack where he exercises his vintage Porsches. For him, driving is as soul-centering an activity as Rees Roberts's hosting of weekend croquet matches ("a blood sport for the British," Harris jokes). But at the end of the day, everyone converges around the fire pit in the outdoor living area designed by David Kelly, a partner in Rees Roberts's firm, or at the two-acre man-made lake whose stone outcropping creates a swimming-hole scene straight out of a Thomas Eakins painting.

That sense of timelessness took a great deal of careful planning. "When we started this house, Lucien and I asked ourselves: 'What do we want to build that in 25 years we'll still be happy with?'" Harris says. "It's a very elemental thing. We've been here 20 years, and we'll be here another 30. You can plant a tree and it will grow, and you can make a lake knowing it will take a decade to settle in."